What can be learned about risk from the results of biomonitoring?

Whether a chemical found in the body poses any risk depends on two factors: (1) the magnitude, time course, and route (ingestion, inhalation or dermal contact) of the exposure, and (2) its toxicity, i.e., what, if any, adverse effects are associated with this type of exposure. Risks can occur if people are very highly exposed for a short time, less highly exposed for a long period of time, or exposed at lower levels to compounds with high toxicity. Thus, knowledge of both toxicity and the characteristics of the exposure is critical in assessing the possible risk.

Since biomonitoring provides only data on exposure, it cannot be used by itself in assessing risk. Monitoring data must be combined with toxicity data if risk estimates are to be calculated. However, it is important to realize that most toxicity data are generated from studies of laboratory animals, usually rats and mice, that are exposed to chemicals in their diet at the same levels every day (often for a lifetime). The results of such research are used to provide estimates of the levels in the diet required to cause adverse health effects in the animals. One problem in combining such results with biomonitoring data is that the relationship between the amount of a chemical in the diet and the amount in fluids and tissues is a complex one. Thus, unless adequate toxicokinetics data are available, it is very difficult to compare these dietary levels used in laboratory experiments to fluid and/or tissue levels measured in biomonitoring studies.

A second problem in combining available toxicity data with exposure data from biomonitoring is that the monitoring results generally reflect the fluid and tissue levels at one instant in time and do not provide information about the time course of the exposure. The monitoring results gathered at one point in time cannot be compared to the toxicity values generated from daily administration of compounds to experimental animals. As a result of these two problems, there is no easy way to use biomonitoring data to assess risk, even if the toxicities of the chemicals being monitored are well understood.

Biomonitoring data and toxicity data can be combined only if scientists have established a connection between particular adverse effects and the levels of a specific chemical in body fluids or tissues. The best example of this is research that has demonstrated the relationship between blood lead levels and neurological problems in young children. Because this information is available, the biomonitoring of children who are likely to have excessive exposure to lead, e.g., children living in older housing with peeling paint, can be used to identify those at risk and thus to develop approaches to reduce exposure and the incidence of adverse effects.

In sum, biomonitoring provides only one part of the data needed to assess risk -- it cannot be used as a surrogate for risk. Even if extensive toxicity data for a chemical are available, they are almost always in a form that is difficult to combine with the biomonitoring-generated exposure values to assess risk. It is only in a small number of special cases, such as that of lead, that inferences about risk can be made readily from biomonitoring data.

Unfortunately, if individuals do not understand the limitations of biomonitoring in providing risk information, they may take steps to reduce exposures that increase rather than decrease their overall risk. A good example is mothers refraining from breast feeding when informed that certain chemicals have been found, or are likely to be found, in their breast milk. In almost all cases, the benefits of breast feeding outweigh any possible risks from these chemicals -- a conclusion that is reflected in the advice given to nursing mothers by public health authorities.